Steel materials, such as high-speed tool steel (Vicker's hardness (HV) of approximately 900) which is tough and whose hardness does not decrease even if the temperature of the steel rises in the 600.degree. C. temperature range, have been used in cutting tools such as turning tools and drills. On the other hand, cemented carbides (HV of approximately 1800), which have superior hardness and resistance to cutting wear since its major component is made from carbides of a metal or metals having a high melting point, have also been used for cutting tools.
Cemented carbide cutting tools made of cemented carbides, are characterized by decreasing the cutting depth while at the same time increasing the cutting speed as much as possible. However, not only are such tools expensive, but they are highly unreliable because they can break suddenly, and therefore applications are limited.
By treating the surface of steel cutting tools made of high-speed tool steel with various surface hardening methods which use gas, plasma, salt baths, etc., a compound layer comprised of iron-nitride, iron-carbide or iron-carbonitride compounds is formed in a few .mu.m thickness on the top surface of the tool, and a layer (this layer is called the surface-hardening layer below) is formed in a thickness from a few .mu.m to a few hundred um under that compound layer, where nitrogen and carbon in atomic state are diffused (solid solution) inside the base material for tool. The hardness of the base material is increased by this surface-hardening layer, and thus improves the resistance to cutting wear of the tool. Accordingly, the surface of the cutting edge portion is hardened.
However, the aforementioned compound layer is brittle. Therefore, methods have been used to remove the compound layer after surface treatment, or to perform surface treatment in conditions that do not allow the compound layer to form. However, if the aforementioned compound layer is removed, part of the super-hard surface-hardening layer is also removed. Also, if surface treatment is performed in conditions that do not allow the compound layer to form, the surface-hardening layer formed is not thick enough.
After the surface-hardening layer has been formed on the steel cutting tool by the above method as a pre-treatment, a duplex surface treatment is then performed for forming a hard coating film. To form this hard film, a method such as the PVD method, which has the advantage of being able to form the film at a relatively low temperature, is used, and a single-layer or multi-layer film of TiN, or TiCN, TiAlN, CrN or the like which is harder and more resistant to oxidation than TiN, is formed. Adhesive ability and durability of the hard coating film are improved by this duplex surface treatment.
For steel cutting tools with a hard coating film formed on them in this way, the hard coating film is subject to chipping and flaking, making it impossible to obtain adequate cutting performance. Therefore, it is necessary to form a surface-hardening layer that is hard and thick enough that it is has the same coating film performance as the hard coating film formed on the cemented carbide.
Methods for forming a nitrogen diffusion layer (surface-hardening layer) or hard coating film having sufficient thickness and good controllability and reproducibility, as well as methods for manufacturing tools using these methods are known (see Japanese patent publications Nos. Tokukai Hei 6-220606, 7-118826, 7-118850, 8-13124, 8-13126, 8-35053, 8-35075 and 8-296064). The aforementioned steel cutting tools have a cutting part that is formed with the following layers, (1), (2), (3) and (4).
(1) Nitrogen diffusion layer on the surface of the steel base material,
(2) (a) a first layer, which is a nitrogen diffusion layer formed on the surface of the steel base material, and (b) a second layer, which is a hard coating film layer formed on the first layer, and which is made of at least one member selected from the group of nitrides, carbides and carbonitrides of at least one member selected from the group of Ti, Zr, Hf, V, Nb, Ta metals and their alloys. The at least one member selected from the group of nitrides, carbides and carbonitrides of at least one member selected from the group of Ti, Zr, Hf, V, Nb, Ta metals and their alloys is hereafter called MN(C) compound.
(3) (a) a first layer, which is a nitrogen diffusion layer formed on the surface of the steel base material, and (b) a second layer, which is a hard coating film layer formed on the first layer, and which is made of at least one member selected from the group of nitrides, carbides and carbonitrides of a Ti--Al alloy. The at least one member selected from the group of nitrides, carbides and carbonitrides of a Ti--Al alloy is hereafter called TiAlN(C) compound.
(4) (a) a first layer, which is a nitrogen diffusion layer formed on the surface of the steel base material, (b) a second layer, which is an intermediate hard coating film layer formed on the first layer and made of an MN(C) compound, and (c) a third layer, which is a hard coating film layer formed on the second layer and made of a TiAlN(C) compound.
In the case of the cutting tools disclosed in the aforementioned patent publications, the nitrogen diffusion layer increases the hardness of the base material, and suppresses deformation of the base material due to local concentrated stresses. Therefore, it prevents chipping of the base material near the cutting edge, and improves the cutting life of the tool. Moreover, if a hard coating film is formed on the nitrogen diffusion layer, the adherence of the nitrogen diffusion layer with the hard coating film is also improved and thus it is possible to suppress flaking of the hard coating film and make a tool that has superior cutting characteristics as well as resistance to wear. In order to sufficiently take advantage of this action, it is best to not form compounds such as iron-nitrides or iron-carbonitrides.
It is possible to use gas nitriding, gas carbo-nitriding, plasma nitriding, salt-bath nitriding or the like as the nitriding method for forming the nitrogen diffusion layer. If compounds are contained in the formed nitrogen diffusion layer, the compounds can be removed by a method such as grinding.
The hard coating film layer that is formed on the nitrogen diffusion layer has a high HV of 1500 to 3000, and it has a small friction coefficient, so it has very excellent resistance to wear.
In the aforementioned hard coating film layer, TiAlN(C) is a substitution type solid solution in which part of the Ti in one or more B1-type crystal structures selected from the group of Ti nitrides, Ti carbides or Ti carbonitrides (hereafter called TiN(C)) is replaced with Al. Moreover, a tight oxide is formed on the surface of the hard coating film made of TiAlN(C) due to the solid solution Al when exposed in an oxidation atmosphere, and it prevents further oxidation of that oxide. Therefore, it prevents degradation due to oxidation of the coating film due to heat generated during cutting.
If the amount of Al is less than 20 mole %, it is not possible to obtain the above action, and if it exceed 70 mole %, the B1-type crystal structure similar to TiN(C) changes and the mechanical properties of the coating film greatly decrease. Therefore, it is best if the amount of Al is between 20 mole % to 70 mole %.
The TiAIN(C) coating film is not as tough when compared with that of TiN(C), since Al exists as a kind of defect. Therefore, when the base material deforms elastically or plastically, it is unable to follow the deformation and it breaks. However, since a nitrogen diffusion layer is formed, it becomes more difficult for elastic or plastic deformation of the base material to occur, so it is possible to suppress breakage. It is better if the hard coating film made of TiAlN(C) is a multi-layer film although it can be a single-layer film. That is because when the toughness of a multi-layer is improved when compared with a single-layer film, so that it contributes to suppressing breakage. This multi-layer film is defined as (1) a film whose Al content changes gradually in the direction of depth, (2) a film whose Al content changes in the direction of depth not gradually but in stages, or (3) a coexistence of both film (1) and film (2).
If an intermediate hard coating film layer (MN(C)) is formed on the nitrogen diffusion layer, the intermediate hard coating film layer is tougher than the hard coating film layer (TiAlN(C)), so that when compared to the case where there is just a hard coating film layer with no intermediate hard coating film layer, the toughness of the overall hard coating film comprised of the intermediate hard coating film layer and hard coating film layer is improved, and contributes to suppressing breakage.
It is best if the thickness of the intermediate hard coating film layer is 90% or less of the thickness of the overall hard coating film. If it exceeds 90%, the thickness of the hard coating film layer is thin (less than 10%), and it is not possible for the function of the hard coating film layer described above (resistance to wear and oxidation) to occur sufficiently.
In order to form the hard coating film layer, as well as the hard coating film layer and intermediate hard coating film layer, a low-temperature film formation method such as a PVD method is best. This is because, in PVD methods such as ion plating or sputtering, it is possible to form the coating film at temperatures below 650.degree. C., and differing from heat CVD methods in which film is formed at high temperature, none of the Nitrogen diffusion layer is lost due to heat. Moreover, it is possible to produce a coating film that has strong bonding strength effective in improving the resistance to sliding friction wear.
Steel cutting tools, the cutting part of which has been treated with a surface treatment such as described above, and which has a sharp cutting edge, are used in the following cutting conditions; cutting speed: 1 m/min to 200 m/min., depth of cutting: 0.1 mm to 20 mm, and feed: 0.01 mm to 10 mm. If used under these conditions, the steel cutting tool has excellent cutting characteristics.
However, as mentioned above, depending on the operating conditions, wear or chipping of the cutting edge occurs easily, and in the conventional steel cutting tools, there has been the problem that it is not possible to take full advantage of the various strong points of the aforementioned surface treatment.